deprecate registration_callback_t in favor of const char* version

- Introduced a new typedef: registration_callback_v2_t, which uses const char*
  instead of std::string for domain and operator names.
- Updated MasterRegistry::register_function to include a deprecated overload that forwards
  std::string parameters to the new const char* version.
- Updated loadSharedLibs() to support both v2 and legacy v1 custom operator libraries.
  It attempts to resolve the v2 interface first, and falls back to v1 if necessary.

Change-Id: I9c0d7b768c1b9be5daeaf9617c6fd46000a32758
Signed-off-by: Peng Sun <peng.sun@arm.com>